When performing macro inspection of integrated circuit wafers, several factors contribute to its effectiveness under varying circumstances. Included among these factors are the illumination angle, illumination intensity, angle of detection, polarization. The illumination and detection angles in particular affect observation in many ways. Brightfield observation occurs when the detected light is specularly reflected from the sample surface, i.e., when the angle of illumination and the angle of detection are the same. Brightfield inspection is useful for inspecting patterned regions with shallow topography, i.e., not much z variation. On the other hand, dark field inspection detects scattered and diffracted light, rather than specularly reflected light. Dark field inspection is often more effective when small changes are being detected, since a small change causes a larger percentage change of the less intense scattered darkfield signal.
In the case of integrated circuit inspection, the metal lines on the circuit in general approximate a grating configuration, so that scattered light usually forms a diffraction pattern. The grating equation applies:sin(θ)+sin(θi)=nλ/D where λ, is the wavelength of incident light, D is the grating spacing, θi is the angle of incidence of light with respect to a surface normal, θ is the angle of detection of light, and n is the diffraction order. FIG. 1a illustrates a resulting diffraction pattern in side view. Incident light beam 100 from illumination source 105 is at angle θi from normal onto sample 110. Zeroeth order diffracted light beam 115, i.e., specularly reflected beam is shown at angle θi, as well as first order diffracted beam 120 centered at angle θ1 and second order diffracted beam 125 centered at angle θ2. The diffracted beams can be thought of as narrow “lobes” centered at the respective angles. The lobes typically decrease in intensity as the diffraction order increases. For detecting small changes in the sample structure, e.g. defocus effects, it is preferable to observe the higher order diffraction lobes, which will show the largest percentage change.
As the grating spacing D decreases, i.e. integrated circuit dimensions decrease and packing density increases, the diffraction lobes separate and spread out. As D approaches λ, it is necessary to have a larger angle of incidence in order to see higher order diffraction lobes. Also, the higher order diffraction lobes will appear at larger angles away from normal as D decreases, therefore the detection angle must also increase. This can be demonstrated as follows:
Assume that D=2λ, and assume normal incidence light, i.e., θi=0. Then the grating equation becomes sin(θ)+sin(0)=n/2, or sin(θ)=n/2. In this case, shown in FIG. 1b, the first order diffraction lobe 130 (n=1) will be centered at a 30 degree angle from normal to the sample surface, and the second order diffraction lobe 135 will extend parallel to the sample surface, nearly impossible to detect. Assuming grazing incidence light, i.e., θi=90 degrees, then the grating equation becomes sin(θ)+sin(90)=n/2, or sin(θ)=n/2−1. In this case, shown in FIG. 1c, the first order diffraction lobe 140 will be at a −30 degree angle from normal to the surface, (i.e., in the same quadrant as the incident light), the second order diffraction lobe 145 will be normal to the surface, the third order diffraction lobe 150 will be at a +30 degree angle from normal, and the fourth order diffraction lobe 155 will extend parallel to the sample surface.
Clearly, then, using oblique incidence light, and being able to tune the angle of detection to be a variable off-normal angle, yield advantages for darkfield sample inspection when higher order diffraction lobes are preferable, as in the case when inspecting for defocus defects. The ability to vary both the angle of incidence and angle of detection of light would provide the maximum flexibility to optimize inspection according to the details of the sample. The use of oblique incidence additionally yields a strong polarization dependence which is not present for normal incidence light. This polarization dependence, which increases as the angle of incidence increases away from normal, is further described in commonly authored and owned U.S. patent application Ser. No. 10/829,727, filed Apr. 22, 2004, issued as U.S. Pat. No. 7,142,300, on Nov. 28, 2006, which is hereby incorporated by reference. This effect provides substantial background suppression, allowing improved inspection of small changes in signal.
A manual inspection system incorporating a tiltable, rotatable table for mounting the sample on so as to provide a wide range of incidence angles for brightfield inspection is described in U.S. Pat. No. 5,096,291, issued Mar. 17, 1992. An inspection system using diffracted light is described in U.S. Pat. No. 5,777,729 (assigned to Nikon Corp.), issued Jul. 7, 1998. As described therein, (and as implemented in the Nikon AMI inspection system), the wafer to be inspected is mounted on a tiltable, rotatable plate so as to be able to tune the inspection angle. The wafer motion during inspection causes problems in matching images. Furthermore, the large parabolic mirrors used for collecting outgoing light have a large focal length, resulting in a very large inspection machine, which costs valuable fab space.
An extension of darkfield detection known as “double darkfield” occurs when the incident light beam and the scattered beam which is detected are not co-planar. In the most pronounced case, illustrated in FIG. 2, the xz plane 205 formed by the incident beam 210 and the sample normal 212 is perpendicular to the yz plane 215 formed by the detected scattered beam 220 (by detector 225) and the sample normal. In this case, the detected light has been scattered in two directions, which provides for a very low background intensity upon which small changes are quite pronounced.
A method of incorporating oblique incidence illumination and double darkfield capability, as well as broad tunability of angle of incidence and angle of detection, into a line scan macro-inspection system such as the Viper system by KLA-Tencor (described in U.S. Pat. No. 5,917,588 by Addiego, which is hereby incorporated by reference), would be a significant advancement in macro-inspection technology. (The current Viper system has limited oblique incidence and detection for brightfield, but with angle limited to being fairly close to normal, i.e., approximately + or −30 degrees, so as to not interfere with the motion stage). The additional ability to incorporate multi-channel detection would allow the most complete inspection according to multiple parameter variations.